@dswd depends on the project. If you’re creating a WP theme and want to make it compatible with BuddyPress, simplest requirements are: the_title() and the_content() and using jQuery which comes bundled in WP. That’s it. BuddyPress is compatible with nearly all WordPress themes.

Given that BP 1.7 introduced theme compatibility, and 1.8 will extend that considerably as we have now brought into play a new level of template hierarchy along with placement of assets CSS/JS in BP directories the question is moot, as you can do either but a child theme is always a child of whatever parent theme you have chosen to use – unless you decide to overload BP by adding bp directories directly to the top level of a theme in which case theme compatibility would be automagically disabled.

The rule of thumb is that if your chosen theme can be updated and you want to make custom changes to BP template files then you do so in a child of that parent theme, even if only containing the ‘buddypress’ ( or ‘community’ ) directory to ensure your changes are maintained in your own child theme.